1. Field of the Invention
The present invention relates broadly to wet chemical methods of depositing gold films on non-metallic substrates. More particularly, this invention relates to a method for the rapid deposition of very thin uniform transparent gold films on transparent substrates, such as glass.
2. Description of the Prior Art
The art of formation of gold films on various surfaces has a long history. Ancient alchemists discovered methods for depositing gold by chemical reduction. Metals were particularly adaptable to being coated with gold films and many articles were decorated in this manner.
The excellent reflective properties of gold films were later utilized in the art of making gold mirrors, especially when it was discovered that gold films could be made to adhere to non-metallic substrates such as glass. However, the cost of gold has restricted the use of gold mirrors to very limited decorative purposes. The earlier history of gold films and the development of the art of depositing bright gold films on non-metallic substrates are discussed at length in a series of articles by Samuel Wein entitled "GOLD FILMS", appearing in THE GLASS INDUSTRY, a trade journal, in 1959. Major disadvantages encountered in the described methods were bulk precipitation of reduced gold metal and a very slow rate of deposition, requiring up to 24 hours of immersion of the article in the plating solution. Other references disclose methods for depositing gold films involving relatively complicated process steps or requiring elevated temperatures.
Aufsberg describes a method in German Patent No. 178,523 for effecting a cohesive, brightly reflecting gilding on a substrate such as glass by first providing the surface with a thin non-reflective film of silver or silver which contains some lead. Shortly before contacting the sensitized surface, the alkaline gilding solution, comprising a gold salt and sodium chloride, is mixed with small amounts of a strong reducing agent such as hydrogen peroxide, sodium peroxide, formaldehyde, ether, and so on, or with a mixture of these substances. This gilding process results in a bright, cohesive gold mirror. The method also teaches that for gilding a flat surface, such as a glass plate, a moldable gutta-percha rim should be provided to hold the solution in place, since it is necessary for the solution to remain in contact with the surface for a prolonged period.
A more recent development in the art of depositing gold films appears in U.S. Pat. No. 3,300,328 to Luce. The invention is described as an aqueous electroless gold plating bath comprising a gold compound, an ammonium or alkali metal sulphite or metabisulphite complexing agent, and a hydrazine or hydroxylamine reducing agent. Like the Aufsberg method previously described, the method of depositing a gold film taught by Luce involves first depositing a thin metal film in order to accomplish sensitization when a non-metallic substrate is employed. However, the strength of the reducing agents disclosed by Luce promotes deposition of a gold film at higher rates than had been previously encountered. While the Luce method for depositing gold films is relatively fast when compared with the Aufsberg method, requiring as little as 40 minutes, it has the disadvantage of requiring elevated temperature and agitation of the bath in order to maximize the rate of deposition.
Another more recent development in the art of depositing gold films is a method described in British Patent No. 1,257,995 to Albeck for gold-plating circuit substrates. This method uses sensitization and activation processes now considered standard in the electroless coating art, followed by a gold-plating process performed in a bath containing chloroauric acid, urea and citric acid. Suitable gold layers are deposited in only 30 to 60 minutes; however, this method also requires elevated temperatures in addition to cooling and filtering apparati. Nonetheless, this invention makes advantageous use of the electrical conductivity of gold films.
More recently, the heat-reflective character of gold films has commanded more attention than their electrical conductivity. Solar control properties have become increasingly important in coatings for glass used in architectural applications. Such coatings are expected to give aesthetically pleasing color results in addition to reflecting solar energy to aid in controlling the environment within the enclosed structure.
Recent advances in this field have been made with films of a variety of metals. Transparent metal films have been produced on transparent articles by a variety of techniques, including electroless or wet chemical techniques. These techniques generally involve contacting the article to be coated with solutions suitable for depositing a metal film thereon by reducing a metal salt from the solution. Typical of the successful methods are U.S. Pat. No. 3,671,291 to Miller and Cavitt for the deposition of nickel, iron and cobalt and U.S. Pat. No. 3,457,138 to Miller for the deposition of copper. For such techniques to be commercially practicable, it is necessary that they be relatively simple, rapid, and readily adaptable to the glass production line. Known techniques for the deposition of gold films are generally inadequate.
The previously described method of Luce is relatively simple. However, in addition to the fact that the method is too slow to be conveniently adaptable to continuous production line coating, the gold films deposited thereby are not transparent and therefore unsuitable for use in conjunction with glass substrates for architectural applications where an aesthetically pleasing uniform transparent article is required.
A sufficiently rapid method for the deposition of gold films is described by Levy in U.S. Pat. No. 3,515,571. According to this invention, gold films may be deposited on various metallic and non-metallic substrates by contacting a surface with a solution of a gold salt and a coordinating ligand and another solution containing a hydrazine compound as the reducing agent for gold. Suitable films can be deposited in less than one minute at room temperature. A useful property of gold films deposited by this method is their electrical conductivity, and Levy suggests their use as conductors, electrodes and contacts. An especially useful application of the method, taught by Levy, is the spraying of gold films on glass. Rapid deposition of specular films is taught for the production of high quality gold mirrors. However, specular films are not suitable for architectural applications wherein viewing through the coated article is a desired object.
The method of Kushihashi described in U.S. Pat. No. 3,484,263 provides a homogeneous coating on glass resulting in an article having high thermal reflectance. However, this method is not sufficiently simple to allow its adaptation to continuous, production-line coating because it requires exposure to radiation in order to promote the reduction of the gold salt to gold. In addition, the resultant gold films are semi-transparent and therefore unsuitable for many applications wherein a transparent article is required or desirable.
Finally, a method of depositing a transparent gold film is described by Soderberg in U.S. Pat. No. 3,476,594. After standard processes of sensitization and activation of the substrate are completed, a nickel film is deposited thereon by a conventional known technique. A gold film is then deposited over the nickel film, preferably from a solution containing potassium gold cyanide at an elevated temperature. Deposition of the gold film is followed by heat treatment to decrease the surface resistance and heat transmittance and increase the light transmittance. The resultant articles possess a combination of desirable properties. The nickel film imparts a neutral color to transmitted light while the gold film renders the article electrically conductive and heat-reflective. While the final product has desirable properties, the multi-step process is undesirable when adaptation to continuous production-line coating is considered.
The present invention provides a fast, simple, and commercially-practicable method of depositing uniform, transparent gold films onto glass to provide aesthetically pleasing transparent articles having excellent solar energy control properties.